1. The Field of the Invention
The present invention relates to methods and apparatus for controlling blood gas levels. More particularly, the present invention is directed to methods and apparatus for monitoring and adjusting the concentration of oxygen and carbon dioxide dissolved in blood, particularly during surgery and other medical procedures.
2. The Prior Art
In order to maintain biological activity the tissues of the body require various nutrients, and particularly oxygen. As a byproduct of this biological activity, various waste products, including carbon dioxide, are formed and must be removed from the body. The blood serves as a carrier of these nutrients to and waste products from the body tissues.
Under normal circumstances, the lungs serve to replenish oxygen to the blood and to remove excess carbon dioxide therefrom. This is accomplished because of the difference between the partial pressure of oxygen (hereinafter sometimes referred to as "pO.sub.2 ") and the partial pressure of carbon dioxide (hereinafter sometimes referred to as "pCO.sub.2 ") in the lungs and in the blood passing therethrough.
Typically, pulmonary arterial blood (the blood being pumped to the lungs for gas exchange) has a pO.sub.2 of about 40 millimeters of mercury ("mmHg") and a pCO.sub.2 of about 45 mmHg. However, the typical partial pressures of oxygen and carbon dioxide in a person's lungs are about 104 mmHg and 40 mmHg, respectively. As a result of this large difference in the partial pressure of oxygen in the lungs in comparison to that in the pulmonary arterial blood, there is a great tendency for oxygen to diffuse through the respiratory membrane of the lung alveoli into the blood. Although there is a much smaller difference between the partial pressure of carbon dioxide in the lungs and in pulmonary arterial blood, carbon dioxide diffuses much more readily than does oxygen, and thus there is also a great tendency for carbon dioxide to pass through the respiratory membrane from the blood and into the alveoli, from which it is expired. The gas exchange which takes place in the lungs is very efficient, and blood passing through the lungs typically attains a pO.sub.2 and pCO.sub.2 equal to that of the air in the lungs, that is, a pO.sub.2 of about 104 mmHg, and a pCO.sub.2 of about 40 mmHg.
Once oxygen dissolves in the blood, it quickly combines with hemoglobin contained in erythrocytes (also known as "red blood cells"). As the blood is then pumped to body tissues having a low oxygen content, oxygen dissociates from the hemogloblin and diffuses into the tissues. At the same time, excess carbon dioxide diffuses from the tissues into the blood, where about one-third of the carbon dioxide combines with hemoglobin, about half is converted to bicarbonate ion, and the remaining one-sixth remains as carbon dioxide dissolved in the blood.
The bicarbonate ion is extremely beneficial when maintained in approximately the proper concentration because it serves to buffer the pH of the blood to a pH of 7.4, which is the optimum pH for biological activity of many enzymes contained in the body. Deviating from this normal pH alters the normal metabolic functioning of these enzymes and can result in trauma or even death.
Because about half of the carbon dioxide dissolved in blood is present as bicarbonate ion, the pH of the blood is directly dependent upon the pCO.sub.2. Thus, an increase in pCO.sub.2 leads to a lowering of the pH of the blood, a condition known as "acidosis," and a decrease in pCO.sub.2 leads to a rise in blood pH, a condition known as "alkalosis." As indicated above, either condition can lead to serious consequences because of the alteration caused in normal metabolic functioning of critical enzymes. Thus, it is important that the pCO.sub.2 be kept within narrow limits, preferably within the range of about 35-45 mmHg.
The pO.sub.2, on the other hand, is less critical. Under normal circumstances, as stated above, the pO.sub.2 fluctuates broadly between about 104 mmHg after the blood has passed through the alveoli, and about 40 mmHg after the blood has made a circuit through the body. The pO.sub.2 can actually drop to about 25 mmHg before the tissues begin receiving inadequate amounts of oxygen. At the same time, it is not dangerous to increase pO.sub.2 above normal levels unless those normal levels are greatly exceeded. Thus, it is believed that PO.sub.2 can, under certain circumstances, be raised to as much as about 300 mmHg without significant adverse affect.
During open heart surgical procedures, it is necessary to establish cardiopulmonary bypass in order to mechanically perform the functions normally produced by the heart and lungs. The treatment of blood extracorporeally and the administration of such treated blood to a patient is known as blood perfusion. Proper blood treatment that will result in adequate perfusion to a patient undergoing cardiopulmonary bypass requires careful balancing and adjustment of the concentration of oxygen and carbon dioxide dissolved in the blood.
Several factors are important in selecting an appropriate pO.sub.2 for a given patient. For instance, under normal circumstances red blood cells make up about forty to forty-five percent (40-45%) of the blood. The percentage of red blood cells is often referred to as the hematocrit, hence, forty percent (40%) red blood cells means the blood has a hematocrit of forty (40). On the other hand, for a number of reasons, the hematocrit of a patient undergoing cardiopulmonary bypass is typically only in the range of about fifteen to thirty (15-30). Thus, since the decrease in hematocrit means that there is a corresponding decrease in the amount of hemoglobin, it is necessary to increase the pO.sub.2 to insure that a sufficient amount of oxygen is available to the body.
At the same time, it is quite common during cardiopulmonary bypass for the surgeon to cause the temperature of the patient to be reduced significantly from the normal body temperature of 37.degree. C. It is known that for every 7.degree. C. that the body temperature is reduced, the metabolism decreases by about fifty percent (50%). As the metabolism decreases, so does the need for oxygen; hence, a lower pO.sub.2 than normal will be sufficient to maintain biological activity.
Yet another important factor in selecting an appropriate pO.sub.2 for blood returned to a patient after extracorporeal treatment is the flow rate of the blood; a decrease in the blood flow rate will necessitate an increase in the amount of oxygen carried by a given volume of blood, and an increase in blood flow rate will permit use of a lesser amount of oxygen carried by the same volume of blood.
Accordingly, it will be appreciated that the selection of an appropriate pO.sub.2 is dependent upon many factors, and more properly, it is the amount of oxygen actually utilized by the body and the efficiency of oxygen transfer during perfusion that is important. During cardiopulmonary bypass, a pO.sub.2 of arterial blood (the blood being returned to the body) in the range of about 100-200 mmHg is required to insure adequate oxygen transfer and to maintain the pO.sub.2 of venous blood (that leaving the body for extracorporeal treatment) in the desired range of about 25-40 mmHg.
During the course of surgery, both venous and arterial blood samples are regularly sent to the laboratory for analysis. Typically, the information obtained are values for pH, pCO.sub.2, pO.sub.2, and hematocrit. Based on this information, the blood perfusionist can determine the adequacy of perfusion and thus make appropriate modifications to the extracorporeal blood treatment apparatus.
Unfortunately, it is time-consuming to send blood samples to the laboratory for analysis, and it may be as long as thirty minutes before the perfusionist obtains the test results. After analyzing these results, making what is hoped to be the proper adjustments, and then allowing the patient to stabilize for a few minutes, a new blood sample must be taken and analyzed. It is common to undercorrect or overcorrect on the basis of previous test results, yet be unaware of the improper adjustment for an additional thirty minutes. This procedure can, of course, result in serious harm to a patient. This is of particular concern when it is realized that a patient undergoing cardiac surgery is already undergoing substantial trauma and the additional trauma imposed by improper perfusion may have extremely serious consequences.
From the foregoing, it will be readily apparent that it would be a significant advancement in the art of blood perfusion to provide methods and apparatus that would provide more timely and precise control over the blood gas levels during cardiopulmonary bypass despite the variation in hematocrit, body temperature, and blood flow rate through the perfusion apparatus. Such apparatus and methods are disclosed and claimed herein.